#!/usr/bin/perl -w
my $file_name = "test.txt";
open(INF,'./$file_name') || die "file could not open\n";
open(INFN,'>./${file_name}.new') || die "$!: ${file_name}.new\n";
my @line;
while(@line = <INF>) {
        if ( $line =~ /test/) {
            print "found at $.: $line
            $line = "\# $line";
        }
        print INFN $line;
}
close(INF);
close INFN;